This invention concerns low-pressure metal casting in a multi-stage recessed sand mold. It further concerns a method and apparatus for feeding several impressions of a single stage of such a mold with low-pressure casting metal, the feed occurring through a casting chamber via at least one intermediate duct and ingates.
A low-pressure casting method (see, for example, commonly assigned French Patent Nos. 2,295,808; 2,367,566; and 2,556,996) is particularly advantageous, when compared with gravitational casting, for the production of thin-walled metal parts and/or parts having complex shapes and/or parts of large size. In fact, the pressure exerted by the metal, which results from the injection of a gas inside a water-tight cavity containing the molten metal, may be closely and accurately controlled to push the metal into all of the innermost recesses of the impressions.
In conventional techniques, at each stage of the mold, a single intermediate duct, or two such ducts positioned diametrically opposite each other, connect the casting chamber to an entire set of ingates belonging to the stage. This method exhibits the following disadvantages linked to the large cross-section of the duct(s):
(1) When the parts to be cast have a complex elongated shape and their impressions are arranged head to tail at each stage of the mold, the ingates are not arranged symmetrically in relation to the casting duct which feeds them, and the number of ingates for a given duct may vary from one impression to another at the same stage. Thus, the filling of two or more impressions in a given stage is not balanced.
(2) The single duct has a large cross-section promoting turbulence in the metal flow and, consequently, the erosion of the sand and occlusion caused by air bubbles, thus compromising the integrity of the parts obtained; furthermore, a duct of this kind consumes a large quantity of metal when filled.
(3) In the case of a multiple-stage mold, because of the large section of the intermediate duct, it is not possible to cause the metal to rise rapidly to the top of the casting chamber, and filling actually occurs stage by stage, a phenomenon which makes it impossible to profit from all of the advantages of low-pressure casting.
(4) When the pressure is lowered and after the solidification of the ingates, which thus form obturators (see the aforementioned French Patent No. 2,295,808), the metal contained in the intermediate duct, which constitutes a relatively large volume that has cooled appreciably, returns to the casting cavity. During the following casting operation, the cooler metal is the first to rise into the casting chamber, thus adversely affecting the quality of some molded parts. For the same reason, an excessive metal flow is required during each casting.